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Is the bootloader replaced when IOS is upgraded?

I know there is ROM that can boot the switch or router without an IOS to ROMMON prompt. Can that ROM be upgraded? I assume not because of the name of "Read Only Memory"

Is the bootloader part of the IOS? If so then why is the version number different than the IOS? Can the bootloader be updated by itself?

I understand the boot system command but I am curious why different version numbers between bootldr and IOS?

switch1#sh ver
Cisco IOS Software, C3750 Software (C3750-IPSERVICESK9-M), Version 12.2(53)SE1, RELEASE SOFTWARE (fc2)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2010 by Cisco Systems, Inc.
Compiled Fri 12-Mar-10 15:50 by prod_rel_team
Image text-base: 0x01000000, data-base: 0x02F00000

ROM: Bootstrap program is C3750 boot loader
BOOTLDR: C3750 Boot Loader (C3750-HBOOT-M) Version 12.2(44)SE5, RELEASE SOFTWARE (fc1)

switch1 uptime is 13 weeks, 6 days, 12 hours, 52 minutes
System returned to ROM by power-on
System restarted at 20:12:53 PDT Tue Sep 2 2011
System image file is "flash:c3750-ipservicesk9-mz.122-53.SE1.bin"

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cisco WS-C3750G-12S (PowerPC405) processor (revision N0) with 131072K bytes of memory.
Processor board ID xxxxxxxxxxx
Last reset from power-on
2 Virtual Ethernet interfaces
12 Gigabit Ethernet interfaces
The password-recovery mechanism is enabled.

512K bytes of flash-simulated non-volatile configuration memory.
Base ethernet MAC Address       : xx:xx:xx:xx:xx:xx
Motherboard assembly number     : 73-9678-05
Power supply part number        : 341-0048-03
Motherboard serial number       : xxxxxxxxxxxx
Power supply serial number      : xxxxxxxxxxxx
Model revision number           : N0
Motherboard revision number     : A0
Model number                    : WS-C3750G-12S-E
System serial number            : xxxxxxxxxxxxxx
Top Assembly Part Number        : 800-26634-02
Top Assembly Revision Number    : C0
Version ID                      : V04
CLEI Code Number                : CNM8ZW0GRB
Hardware Board Revision Number  : 0x06

Switch Ports Model              SW Version            SW Image
------ ----- -----              ----------            ----------
*    1 12    WS-C3750G-12S      12.2(53)SE1           C3750-IPSERVICESK9-M

Configuration register is 0xF

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1 Solution
Garry GlendownConsulting and Network/Security SpecialistCommented:
The boot loader usually just contains the core features in order to access flash and get the system booted. On newer system, it may also include boot features to retrieve the regular IOS from a server, e.g. via TFTP. On top of that, recovery of the system is available, either via TFTP oder serial upload.
In most cases, the boot loader does not need to be touched. Anyway, there may be occasions in which an update is required, e.g. to support some new piece of hardware. I'm not sure whether all Cisco gear allows for boot "rom" upgrade, but for some systems like e.g. the 7200 series, the boot IOS can be replaced (check out the "bootflash:" device). Anyway, those systems do use a two-stage boot sequence, one for something you could compare to the BIOS of a PC, and then the boot IOS.
Dragon0x40Author Commented:
I am still confused about this.

If I am told to upgrade the firmware and/or IOS what exactly does that mean.

I believe that the legacey CATOS had a seperate boot file in flash to assist in booting the CATOS.

Have not seen that in IOS.

Maybe firmware and IOS are interchangable terms but I don't want to make that assumption.

Sometimes the IOS and the bootldr have the same version number and sometimes they don't as in the example I posted.

BOOTLDR: C3750 Boot Loader (C3750-HBOOT-M) Version 12.2(44)SE5, RELEASE SOFTWARE (fc1)

Cisco IOS Software, C3750 Software (C3750-IPSERVICESK9-M), Version 12.2(53)SE1, RELEASE SOFTWARE (fc2)

How would I update the boot loader?
Garry GlendownConsulting and Network/Security SpecialistCommented:
Bootloader upgrade is the same as the IOS upgrade, just that it resides in "bootflash:" instead of the normal flash ...
To upgrade, get the appropriate boot loader image, most likely there's not enough space in the bootflash: partition to store both, so you need to remove the old one (delete bootflash:<FILENAME>), then copy the new image to the bootflash: partition (e.g. from TFTP)
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Dragon0x40Author Commented:
IOS, Firmware and bootloader upgrades are three ways to say the same thing?

Why does the bootloader version not match the IOS version if they are both displaying information about the same IOS image in flash: in a 3700 switch  and bootflash: in a 4500 switch for example?
Garry GlendownConsulting and Network/Security SpecialistCommented:
They are not necessarily the same thing ... depending on the device, you may have a bootrom, a bootloader, and then the main IOS. In case of the 3750, I reckon all three are there ... bootloader and IOS are most definitely different, you can't install one image in place of the other ... most of the larger Cisco boxes (routers, switches) do have separate boot images and operating images ... smaller (fixed configuration) routers usually only have the bootrom (unchangeable) and the IOS.
Dragon0x40Author Commented:
Is firmware and IOS the same thing?

switch 1 and 2 are identical 6500 switches but switch1 has a bootldr but switch 2 does not have a bootldr?

switch2#sh ver
Cisco IOS Software, s72033_rp Software (s72033_rp-ADVENTERPRISEK9_WAN-M), Version 12.2(33)SXI3, RELEASE SOFTWARE (fc2)
ROM: System Bootstrap, Version 12.2(17r)SX5, RELEASE SOFTWARE (fc1)
System image file is "sup-bootdisk:s72033-adventerprisek9_wan-mz.122-33.SXI3.bin"

switch1#sh ver
Cisco Internetwork Operating System Software
IOS (tm) s72033_rp Software (s72033_rp-ADVENTERPRISEK9_WAN-M), Version 12.2(18)SXF16, RELEASE SOFTWARE (fc2)
Image text-base: 0x40101040, data-base: 0x42DDD190
ROM: System Bootstrap, Version 12.2(17r)SX5, RELEASE SOFTWARE (fc1)
BOOTLDR: s72033_rp Software (s72033_rp-ADVENTERPRISEK9_WAN-M), Version 12.2(18)SXF16, RELEASE SOFTWARE (fc2)
System image file is "sup-bootdisk:s72033-adventerprisek9_wan-mz.122-18.SXF16.bin"

Garry GlendownConsulting and Network/Security SpecialistCommented:
I reckon it's just different output, changed from the older 12.2(18) to 12.2(33) version ...
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